In the wood industry, it is common that the wood is treated to obtain certain attributes or features, e.g. resistance to microorganisms, lower contents of natural fluids, altered structural properties, or a particular colour. However, a common and costly problem within wood treatment is warping of the wood, which is explained by two principal effects. Firstly, the warping may be a result of shrinkage anisotropy, resulting in cupping, bowing, and twisting. Secondly, the warping may be a result of uneven drying, leading to structural damage, such as raptures, external and internal checks, and splits
One common step in wood treatment involves heating of a wooden product, which can be achieved by applying different forms of electromagnetic radiation. At the shortest wavelengths, the product is illuminated by infrared radiation, where the heat reaches the interior of the product through convection or conduction from the surface. Microwave radiation can also be applied for heating, where the temperature is increased through direct dielectric heating of the product. This gives a deeper penetration of the applied energy. At the longest wavelengths, the product can be subjected to high-frequency radio emission, which also increases the temperature through dielectric heating, but with a deeper penetration compared with that of microwave radiation, thereby enabling a more homogeneous heating.
For the case of a metal, high-frequency radio emission will induce eddy currents, which will heat the material. This electromagnetic inductive heating is the most efficient if the metal is ferromagnetic, which is the case for several industrial types of steel.
Vacuum drying is another common method in wood treatment, where the product is subjected to dielectric heating. As an example of a general application of vacuum treatment see U.S. Pat. No. 5,575,083. The vacuum lowers the boiling temperature, while the electromagnetic field increases the temperature, resulting in a more efficient drying when combining the techniques.
Another common step within wood treatment involves impregnation with a fluid, e.g. a preservative, in a high-pressure environment. Here, a method is put forward allowing a comparatively large amount of fluid to be added to the structure of the wood by combining steps of heating by electromagnetic radiation, vacuum treatment, and high-pressure treatment.